in a sauce pot over medium heat, melt the butter and then sauté the onions until translucent.
Add in the remaining ingredients, stir to combine, then bring the mix to a boil.
Once boiling, lower the heat to low, then simmer, uncovered, for 30 minutes. Stir occasionally.
Remove from the heat and pour into a blender or food processor, then blend to your desired consistency.
Add salt and pepper to taste.